The present disclosure relates to a positive electrode active material containing a complex oxide including lithium and a transition metal as a constituent element; a positive electrode and a lithium secondary battery using the positive electrode material; and an electric tool, an electric vehicle, and a power storage system using the lithium secondary battery.
Recently, small-size electronic apparatuses such as mobile terminal apparatuses have come into wide use, and there is strong requirement for to be small and light and to have high durability. Accordingly, a battery, particularly, a secondary battery capable of obtaining a high energy density with small size and light weight, as a power supply, is being developed. The secondary battery is not recently limited to the small-size electronic apparatuses, and application of the secondary battery to large-size electronic apparatuses such as vehicles is being studied.
As the secondary battery, it is widely proposed to use various kinds of charge and discharge phenomenon, but attention is being paid to a lithium secondary battery. The reason is because it is possible to obtain a higher energy density than a lead battery and a nickel-cadmium battery. The lithium secondary battery is a lithium ion secondary battery using absorption and discharge of lithium ions or a lithium metal secondary battery using precipitation and dissolution of lithium.
The lithium ion secondary battery has a positive electrode, a negative electrode, and an electrolyte solution, and the positive electrode and the negative electrode include a positive electrode active material and a negative electrode active material absorbing and discharging the lithium ions, respectively. The lithium metal secondary battery has the same configuration as the lithium ion secondary battery except that the negative electrode active material is lithium metal. As the positive electrode active material, a complex oxide including lithium and a transition metal as a constituent element is widely used, but the positive electrode active material directly related to a charge-discharge reaction has a great influence on battery performance, and thus various studies regarding composition of the complex oxide have been made.
Specifically, to improve characteristics of a charge-discharge cycle, it is proposed that a coating of a metal oxide is formed on a surface of a positive electrode including a positive electrode active material represented by LixNi1−yCoyOz (0<x<1.3, 0≦y≦1, and 1.8<z<2.2) (for example, see Japanese Patent No. 3172388). The metal oxide is BeO, MgO, or the like.
To improve structural stability and thermal stability of the positive electrode active material, it is proposed that a metal oxide is coated on a surface of a positive electrode active material represented by LiA1−x−yBxCyO2 (A is Co or the like, B is Ni or the like, C is Al or the like, 0<x≦0.3, and 0≦y≦0.01) (for example, see Japanese Patent No. 3691279). The metal oxide is an oxide such as Mg and Al.
To improve a cycle life and initial capacity, it is proposed that a surface of a spinel-type positive electrode active material represented by LiaMnbMcO4 (M is Mg or the like, 1.0≦a≦1.15, 1.8≦b≦1.94, 0.10≦c≦0.10 and a+b+c=3) is coated with an oxide including a metal element (for example, see Japanese Unexamined Patent Application Publication No. 2009-206047). The oxide including the metal element is an oxide such as Al and Co, and the metal element of the oxide forms a solid body with LiaMnbMcO4.
To improve capacity characteristics, life characteristics, and thermal stability, in a positive electrode active material including an internal bulk portion and an external bulk portion, it is proposed that a metal composition is provided by continuous concentration gradient from an interface between the internal bulk portion and the external bulk portion toward a surface of the active material (see Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-525578). The internal bulk portion is LiNi0.8Co0.13Mn0.07O2 represented by LiaNi1−x−y−zCoxMnyMzO2−δXδ (M is Mg or the like, X is F or the like, 0.95≦a≦1.2, 0.01≦x≦0.5, 0.01≦y≦0.5, 0.005≦z≦0.3, and 0.05≦x+y+z≦0.4). The external bulk portion is LiNi0.4Co0.4Mn0.2O2 represented by LiaNi1−x−y−zCoxMnyMzO2−δXδ (M is Mg or the like, X is F or the like, 0.95≦a≦1.2, 0.01≦x≦0.4, 0.01≦y≦0.5, 0.002≦z≦0.2, and 0.4<x+y+z≦0.95).
To sufficiently utilize high capacity characteristics of an Si-based or Sn-based negative electrode active material, it is proposed that a lithium-rich complex oxide (LihMn1CojNikO2) is used as a positive electrode active material (see Japanese Unexamined Patent Application Publication No. 2009-158415), where h=[3(1+x)+4a]/3(1+a), i=[3α(1+x)+2a]/3(1+a), j=13(1−x)/(1+a), k=γ(1−x)/(1+a), 0<a<1, α>0, β>0, γ>0, α+β+γ−1, and 0≦x<⅓. The complex oxide is a solid solution represented by Li1+x(MnαCoβNiγ)1−xO2.aLi4/3Mn2/3O2.
To improve battery capacity and charge-discharge cycle characteristics, it is provided that an oxide including Li, Ni, and the like is formed on a surface of a positive electrode active material represented by Li1+wCo1−x−yGaxMyO2−z (M is Mg or the like, −0.01≦w≦0.01, 0.0001<x<0.05, 0≦y<0.4, and −0.1≦z≦0.2) (for example, see Japanese Unexamined Patent Application Publication No. 2007-335169).
To obtain excellent cycle characteristics with high capacity and to prevent gas from occurring in a battery at the time of high temperature, it is proposed that a coating layer is provided on a surface of complex oxide particles including lithium and a transition metal (for example, see Japanese Unexamined Patent Application Publication No. 2009-054583). The coating layer includes at least one kind of elements M (different from the transition metal included in the complex oxide particles) selected from the second to the thirteenth groups, and at least one kind selected from the group consisting of elements X of phosphorus, silicon, and germanium, and the elements M and X represent different distribution in the coating layer.